A Japanese submarine made a port call in the Philippines, the first in 15 years, on Sunday in a show of growing military cooperation amid tension triggered by China's growing assertiveness in the South China Sea.

One of the newest and largest submarines in the Japanese navy, it was escorted into the former U.S. Navy Base at Subic Bay by two Japanese destroyers on a tour of Southeast Asia.

"This is just an exercise and the main objective is to train the officers," Captain Hiraoki Yoshino of Japan's Maritime Self-Defence Force told reporters.

"We don't have any message to any country," he said, adding the ship visits were aimed at boosting confidence between the Japan and the Philippines.

China claims almost all the South China Sea, where about $5 trillion of ship-borne trade passes every year. Brunei, Malaysia, the Philippines, Vietnam and Taiwan also have claims.

Japan and China also have conflicting claims over islets in the East China Sea. Japan is increasing its presence in the South China Sea, sending more ships and planes to allies in Southeast Asia, like Vietnam and the Philippines.

The Philippines and the United States start military drills on Monday, including simulating the retaking an island seized by an imaginary enemy in the South China Sea, an exercise likely to rile China.

Japan has offered to help the Philippines boost its capability in monitoring what is going on in the South China Sea by leasing three TC-90 surveillance planes, a deal that could be sealed late this month.